I have a couple of .ePub ebooks and can't get Calibre to convert them to .MOBI. Even the Calibre Reader can't open them and gives me the message:

Traceback (most recent call last):
File "site-packages\calibre\gui2\viewer\main.py", line 40, in run
File "threading.py", line 504, in run
File "site-packages\calibre\ebooks\oeb\iterator\book.py", line 99, in __enter__
File "site-packages\calibre\customize\conversion.py", line 239, in __call__
File "site-packages\calibre\ebooks\conversion\plugins\epub_in put.py", line 161, in convert
File "site-packages\calibre\utils\localunzip.py", line 217, in extractall
File "site-packages\calibre\utils\localunzip.py", line 204, in _extractall
ValueError: Not a ZIP file

Python function terminated unexpectedly
Not a ZIP file (Error Code: 1)
Traceback (most recent call last):
File "site.py", line 132, in main
File "site.py", line 109, in run_entry_point
File "site-packages\calibre\utils\ipc\worker.py", line 186, in main
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 31, in gui_convert_override
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 25, in gui_convert
File "site-packages\calibre\ebooks\conversion\plumber.py", line 1009, in run
File "site-packages\calibre\customize\conversion.py", line 239, in __call__
File "site-packages\calibre\ebooks\conversion\plugins\epub_in put.py", line 161, in convert
File "site-packages\calibre\utils\localunzip.py", line 217, in extractall
File "site-packages\calibre\utils\localunzip.py", line 204, in _extractall
ValueError: Not a ZIP file

When I try to read one of the .ePub books in Sigil, I just get a "cannot open" message. On FireFox's EPUBReader, I get a "format is corrupted." When I submit it to Kindle Convert, it says it can't translate and there might be a DRM problem (It should NOT have DRM).

I'm attaching one of the books for you to check out. I got them from a friend who swears he bought them and I paid him for them, so I had no reason to believe they are mis-formatted until I tried to read and convert them.

If you read though Calibre's messages it reports it's not a zip file, which is inline with the other applications reporting that it's corrupted. Something is wrong with the download, so you should either re-download from the original source, and if that still fails contact the vendor and get them to rectify it.

Something else you could try is to make a copy of the epub file on your hard drive and change the extension to .zip. Then try to unzip it. If it doesn't unzip, that will confirm that it's not a valid epub file. If it unzips, re-zip it (VERY IMPORTANT! Make sure you preserve the directory structure) then change the extension back to .epub and try converting it again.